The present invention relates to a pneumatic tire for high speed and heavy duty use, in which durability of the bead and lower sidewall region is improved, and is suitably applied to airplane tires especially for large-sized airplane.
Recently, even in tires for heavy load and high speed use, e.g. tires for aircraft, radial carcasses came into use in order to improve structural durability, running performance, fuel consumption performance and the like.
In the radial tires for aircraft, the bead portions are subjected to a large bend deformation under heavy load conditions, and internal temperature of the bead portion is greatly increased by high speed running during taking-off and landing of aircraft, which causes bead damages, e.g. separation failure between the adjacent plies and between reinforcement cords and the surrounding rubber.
Even when the running speed is low, if the running time is long, for example during taxiing of aircraft, internal temperature is increased. In this case, in addition to the above-mentioned damages, a rubber breakage extending from the cords to the outer surface of the tire sidewall is caused at a position near the carcass edges.
In order to prevent such failures by reducing the temperature rise, a rubber with a low heat generation property has been used in the bead portion and a lower sidewall portion.
It was, however, found that contrary to our expectations, such a low heat generation rubber tends to induce bead damage at an early stage of the tire life during both a high speed running and a low speed running.
As the result of our various researches, it was found that the distances between the adjacent carcass cords located in the bead portion and a lower sidewall portion influence damage of the bead.
In a rubber with a low heat generation property, a heat generation caused by its bending deformation is small, but its strength is not so large. Therefore, when the above-mentioned distances between the adjacent cords are small, a shear stress generated in the rubber between the cords is increased to cause a separation failure of the carcass cords from the surrounding rubber.
On the other hand, the carcass plies are turned up around the bead cores to be secured thereto as shown in FIG. 3. According to a prior art, the carcass turned up portion A2 is extended along the carcass main portion A1 while contacting directly thereto.
When such a tire is inflated to a high pressure and greatly deflected, in a lower sidewall portion C, a large tensile stress F is worked in the opposite directions between the main portion A1 and the turned up portion A2 of the carcass A. As a result, a large shear exists between the carcass turned up portion and the adjacent carcass main portion.
Further, as the deflection is very large, a compressive stress is generated in the axially outer sidewall portion, and a tensile stress is generated in the axially inner sidewall portion, which further amplifies the shear. And cord end looseness or carcass ply edge separation which extends to a bead damage is liable to be induced at the radially outer edge of the carcass turned up portion A2.
In order to reduce bead deformation, for example, if the thickness and height of bead apexes are increased, the bead volume increases to increase the tire weight, and the heat generation therefrom also increases, which reduces the bead durability.